Sarranids: The Sarranids have a very skilled and versatile, but also weak army.
A good combination of cavalry and archers is all you need to cut down enemy troops like they were made out of pie.
Vaegirs: The Vaegirs have a very balanced army and personally one of my favorites.
Nords are a very infantry-heavy faction with some of the best units of that type in the entire game, the downside however is that they also have high wages and training them can take a long time.
Nords: The Nord-armies are very efficient powerhouses, capable of both dealing and taking a lot of damage.
Swadians: The Swadians have a very strong infantry and cavalry army (though the infantry isn't as good as the Nords) and some units are secretly tagged as having a horse, thus they do not slow you down on the World Map.
=Here is a quick summary of each faction's units: While there are no real story-related reasons or motivations to base your choice on, it is important to know which units are provided by the different factions, seeing as how you are going to be stuck with recruiting them after your choice. Choosing which faction you are going to support is very important in this game, once you pick one, you are instantly doomed to be dragged into every single war they decide to have.